Magnetic field sensors are used in a variety of applications for sensing the rotation of a wheel and/or shaft, such as in anti-lock braking systems, crank-shaft sensors, cam-shaft sensors, etc. Magnetic field sensors are also used for sensing vertical and/or angular movement. Typically, a permanent magnet is attached to the back side of a magnetic field sensor package, which is placed in front of a magnetically permeable tooth wheel. The rotation of the tooth wheel generates a sinusoidally varying magnetic field on the magnetic field sensor. Magneto-resistive (XMR) sensor elements within the magnetic field sensor package detect the component of the magnetic field that is parallel to the surface of the magnetic field sensor package and that points in the direction of the movement of the tooth wheel (i.e., the tangential direction). The detected magnetic field variation provides information about the angular position and rotational speed of the tooth wheel.
The permanent magnet attached to the back side of the magnetic field sensor package has a number of disadvantages. First, the permanent magnet has position tolerances with respect to the magnetic field sensor package since the permanent magnet is typically attached after the completion of the fabrication of the magnetic field sensor package. Second, the glue used to attach the permanent magnet to the magnetic field sensor has to be selected carefully since the sensor assembly is typically exposed to a wide temperature range (e.g., −50° C. to −180° C.).
Third, the permanent magnet is typically attached to the magnetic field sensor package by someone other than the semiconductor manufacturer who fabricated the magnetic field sensor package. Therefore, the permanent magnet is attached to the magnetic field sensor package after the final test of the magnetic field sensor package at the semiconductor manufacturer. The semiconductor manufacturer typically tests the magnetic field sensor package at several temperatures. Thus, the permanent magnet is typically not tested at several temperatures since the thermal mass of the entire sensor assembly is usually too large to perform an economical multi-temperature test.
Fourth, the permanent magnet is typically magnetized after it is attached to the magnetic field sensor package using a large magnetization field. Due to the large magnetization field, there is a risk that the sensor performance will be degraded. This sensor performance degradation is typically not detected before delivery of the sensor. Often, one or more of the pins of the magnetic field sensor package are inaccessible once the entire sensor assembly is fabricated, thus preventing the activation of test modes of the sensor. Finally, for some XMR sensors, such as giant magneto-resistive (GMR) sensors, a typical permanent magnet produces diverging magnetic field or flux lines. The diverging flux lines may drive the XMR sensors into saturation.